This invention relates to an optical logic circuit using a semiconductor laser.
A logic circuit using light as a signal (an optical logic circuit) is considered to have such features that the propagation velocity of light is faster than an electrical signal, and the logic circuit is directly coupled to an optical fiber communication system. In particular, when an optical logic circuit is integrated on a semiconductor substrate, there occur advantages such that the entire dimensions of the circuit are reduced, its reliability is enhanced, and so on. Because of such advantages, intensive research is being concentrated on an optical logic circuit using a semiconductor laser or a semiconductor photodetector.
The following examples, among others, are known: the flip-flop of a light input and a light output using a polarization bistable laser and optoelectronic switch (for example, IEEE Journal of Quantum Electronics, QE-vol. 21, page 298), the optical switch for switching the light output of a tandem type bistable laser from a lower state to higher state by means of an external light input (for example, Proceedings of the 32nd Meeting of Applied Physics Joint Conference, page 140, 31a-ZB-7 to 8), and the logic gate of an electrical input light output using a cleaved coupled cavity laser (for example, IEEE Journal of Quantum Electronics, QE-No. 19, page 1621).
In these methods, however, since an electric current occurs in the signal line, the operation speed is determined by an electrical time constant. To obtain an expected high speed operation by avoiding this, it is necessary to arrange it so that the signal may be entirely processed by light while the current or voltage only supplies bias to the device. Accordingly, a so-called optical bistable device having a hysteresis in the light-output versus light-input characteristic is needed. More particularly, it is preferable for the circuit construction to amplify, more or less, the incident light, which gives rise to the necessity of the development of an optical bistable device using a semiconductor laser which is an active device. An example of an optical logic circuit using a semiconductor laser is described below.
FIG. 11 shows the structure of an example of a conventional optical logic circuit (for example, the one disclosed in the Journal of Applied Physics, No. 56, page 664). This is to make use of a cleaved coupled cavity laser. The cavity lengths L.sub.1 and L.sub.2 of two semiconductor lasers E.sub.1 and E.sub.2 are 250 microns, and the gap d is 5 microns or less, and the active layers of E.sub.1 and E.sub.2 confront each other. The basic structure of E.sub.1 and E.sub.2 is same as that of a buried crescent laser. E.sub.1 denotes an ordinary laser characteristic of a threshold value of 15 milliamperes as indicated by the curve of I.sub.2 =0 milliampere in FIG. 12. E.sub.2 has a similar characteristic. In this setting, when the current I.sub.2 flowing into E.sub.2 is fixed at 21 milliamperes and the current I.sub.1 flowing into E.sub.1 is varied, the light outputs P.sub.1 and P.sub.2 of E.sub.1 and E.sub.2 respectively change as shown in FIGS. 12 and 13. At this time, a hysteresis is observed among P.sub.1, P.sub.2, and I.sub.1 in a region where I.sub.1 is between 23 milliamperes and 32 milliamperes. This phenomenon is considered to be derived from the fact that saturation of the gain has occurred as the laser light from E.sub.1 and E.sub.2 are mutually injected into the other sides. For this reason, the hysteresis has occurred in a region of high light output in which the injection current I.sub.1 is about twice as high as the threshold current. It is characteristic, at the same time, that a hysteresis has also occurred in P.sub.2 although I.sub.2 is constant because the both are strongly bonded optically. The relationship between P.sub.1 and P.sub.2 is expressed by an optical bistability as shown in FIG. 14.
FIG. 15 shows another example of a conventional optical logic circuit device (for example, the one disclosed in the Applied Physics Letters, vol. 41, page 702). In this structure, an ordinary slab type semiconductor laser's P-type electrode is processed into a window type electrode measuring 10 microns in width, 20 microns in length, and 30 microns in pitch. When this element is cooled to 212.1 degrees kelvin and a current is passed therethrough, the light intensity delivered from an active layer 1004 is as shown in FIG. 16. A hysteresis is observed in the vicinity of a current of 153 milliamperes. This is because the p-type electrode is transformed into a window type electrode, so that a non-current injection region is present in an active layer, which functions as a saturable absorber. This device having a bistable property by itself can cause a bistable action also by injecting light from outside. That is, by fixing the current flowing in the device at 152.6 milliamperes, when light is injected by using a buried heterostructure semiconductor laser, an optical bistability is obtained as shown in FIG. 17.
However, in the case of the prior art shown in FIG. 17, although the optical bistability as shown in FIG. 14 may be obtained, light outputs P.sub.1 and P.sub.2 of E.sub.1 and E.sub.2 affect each other, and it is hard to construct such a structure as so to control P.sub.2 by P.sub.1. Besides, because of the utilization of the saturation of the gain of the semiconductor laser, it is necessary to inject a high current, and it is hard to lower the power consumption.
On the other hand, in the case of the prior art shown in FIG. 15, it is necessary to made a saturable absorber within an active layer, it is necessary to operate at a low temperature while elevating the threshold current, which results in slow switching speed, among other problems.